The present invention is generally directed to switched capacitor systems and, in particular, to an amplifier for improving the open-loop gain and bandwidth of a switched capacitor system.
Switched capacitor systems are used in a wide variety of applications, including digital-to-analog converter (DAC) circuits, analog-to-digital converter (ADC) circuits, equalizers, filters, and the like. A switched capacitor system typically comprises an operational amplifier (i.e., op-amp) with a first output (or feedback) capacitor (COUT1) coupled (directly or indirectly) between a first output and a first input of the op-amp and a first input capacitor (CIN1) coupled (directly or indirectly) between the first input and ground. The switched capacitor system also may comprise a second output (or feedback) capacitor (COUT2) coupled (directly or indirectly) between a second output and a second input of the op-amp and a second input capacitor (CIN2) coupled (directly or indirectly) between the second input and ground.
Through a series of switching operations, charge is distributed on these capacitors to effect a closed loop gain. This closed loop gain is subject to a finite error caused by the non-ideal open-loop gain of the operational amplifier. The open-loop frequency response is also attenuated by the capacitive voltage divider produced at the input between the feedback capacitor and the capacitor to ground. The effective capacitance of the capacitor to ground is equal to the sum of the capacitance of the fixed external capacitor and the input (parasitic) capacitance of the operational amplifier. Generally, the input parasitic capacitance is predominantly caused by the gate-to-source capacitance of MOSFET devices in the operational amplifier. Hereafter, the input parasitic capacitance may be approximated by using the gate-to-source capacitance interchangeably.
In any given system, reduction of the op-amp input capacitance reduces the capacitance voltage division at this node and improves the open-loop gain and bandwidth of the system. Reducing the op-amp input capacitance may be achieved by making the input transistors of the op-amp smaller. However, reducing op-amp input capacitance is problematic when dealing with very small devices. Very small channel length devices typically have higher gains than larger channel devices. However, small channel length devices (i.e., 0.18 micron) are unable to withstand large operating voltages. Hence, small channel length devices typically operate from a 1.6 volt to 2.0 volt power supply, while larger channel length device (i.e., 0.4 micron) typically operate from a 2.7 volt to 3.6 volt power supply. Thus, reducing the device size in order to reduce input capacitance often entails reducing the power supply. Since a reduced power supply results in reduced performance of the op-amp transistors, at least part of the benefit of reducing input capacitance is lost.
Therefore, there is a need in the art for an improved operational amplifier for use in switched capacitor systems. In particular, there is a need for an operational amplifier having a reduced input capacitance that is still able to operate from higher power supply voltages.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an operational amplifier that has both thick-oxide transistors and thin-oxide transistor and operates from a high voltage supply. The present invention replaces a thick oxide transistor with a thin oxide transistor having smaller width (W) and length (L) at the input of the op-amp. Since the capacitance of the oxide (Cox) is inversely proportional to the thickness of the oxide (Tox), the thin-oxide input transistor would actually have increased input parasitic capacitance per square micron, if all other factors are the same.
To maintain the same amplifier characteristics, the W/L ratio of the transistors may be held constant. However, W and L are reduced proportionally. If the product, Wxc3x97Lxc3x97Cox, is lower for the thin oxide device, then replacing a thick-oxide transistor with a thin-oxide transistor having much smaller area decreases the overall input capacitance of the op-amp, thereby improving open-loop gain and bandwidth.
As an example, consider an operational amplifier that has two device (i.e., transistor) types. The first device has a minimum L=0.4, Tox=70 A, and a Cox=5 fF/um2. The second device has a minimum L=0.2, Tox=35 A, and a Cox=10 fF/um2. For a given W=40 and L=0.4, the first device has a parasitic capacitance, Cp1 (e.g., approximately equal to gate-to-source capacitance, Cgs), of 0.4(40) (5 fF)=80 fF. If the second is scaled to obtain the same W/L ratio, with minimum L=0.2 and W=18, then the second has a parasitic capacitance Cp2=Cgs=0.2(18)(10 fF)=40 fF. Assume the output (feedback) capacitor, COUT, is 80 fF and the input capacitor to ground, CIN, is 80 fF. Therefore, the voltage division at the input of the op-amp is 80/160=0.5 for the first device. The voltage division at the input of the op-amp is 80/120=0.67 for the second device. This represents an improvement of 1.34 over the first device (or a 2.5 dB improvement in open-loop gain). Similarly, bandwidth is improved.
The present invention is able to use thin-oxide devices in an operational amplifier operating from a large power supply by replacing only devices that are not exposed to the full power supply voltage. For example, replacing some of the N-type transistors in an input differential pair of the op-amp allows such a switched-cap amplifying system to obtain improved open-loop gain and bandwidth using thin oxide devices while maintaining a power supply voltage suitable for use with the thick oxide devices. The biasing of the cascade transistors and other devices in the operational amplifier must be such that the thin-oxide transistors in the differential pair never see more than, for example, a 1.8 volt gate-to-source difference, gate-to-drain difference, or gate-to-bulk difference.
Thus, according to an advantageous embodiment of the present invention, there is provided an amplifier capable of operating from a power supply having a first voltage level, wherein the amplifier comprises: 1) a plurality of thick-oxide field effect transistors, each of the plurality of thick-oxide field effect transistors having a relatively thick oxide layer and fabricated using a first process such that the each thick-oxide field effect transistor is capable of withstanding a gate-to-source difference and a gate-to-drain difference at least equal to a first maximum operating voltage, wherein the first maximum operating voltage is at least equal to the first voltage level; and 2) a first thin-oxide field effect transistor coupled to a first input of the amplifier, the first thin-oxide field effect transistor having a relatively thin oxide layer and fabricated using a second process such that the first thin-oxide field effect transistor is capable of withstanding a gate-to-source difference, a gate-to-drain difference, and a gate-to-bulk difference at least equal to a second maximum operating voltage, wherein the second maximum operating voltage is less than the first voltage level.
According to one embodiment of the present invention, a configuration of the plurality of thick-oxide field effect transistors and the first thin-oxide field effect transistor prevents the first thin-oxide field effect transistor from being exposed to a gate-to-source difference greater than the second maximum operating voltage.
According to another embodiment of the present invention, the configuration of the plurality of thick-oxide field effect transistors and the first thin-oxide field effect transistor prevents the first thin-oxide field effect transistor from being exposed to at least one of a gate-to-drain difference and a gate-to-bulk difference greater than the second maximum operating voltage.
According to still another embodiment of the present invention, the amplifier further comprises a second thin-oxide field effect transistor coupled to a second input of the amplifier, the second thin-oxide field effect transistor having a relatively thin oxide layer and fabricated using the second process such that the second thin-oxide field effect transistor is capable of withstanding a gate-to-source difference, a gate-to-drain difference, and a gate-to-bulk difference at least equal to the second maximum operating voltage.
According to yet another embodiment of the present invention, a configuration of the plurality of thick-oxide field effect transistors and the second thin-oxide field effect transistor prevents the second thin-oxide field effect transistor from being exposed to a gate-to-source difference greater than the second maximum operating voltage.
According to a further embodiment of the present invention, the configuration of the plurality of thick-oxide field effect transistors and the second thin-oxide field effect transistor prevents the second thin-oxide field effect transistor from being exposed to at least one of a gate-to-drain difference and a gate-to-bulk difference greater than the second maximum operating voltage.
According to a still further embodiment of the present invention, the first and second thin-oxide field effect transistors comprise a differential pair input stage.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.